Nodular cast iron containing silicon and vanadium

ABSTRACT

AN IMPROVED CAST IRON PARTICULARLY ADAPTED FOR USE IN ARTICLES OCCUPYING A HIGH TEMPERATURE ENVIRONMENT AND COMPRISING ABOUT 3.50% TO 5.00% SILICON; ABOUT 0.70% TO 2.00% VANADIUM; AND ABOUT 2.80% TO 3.80% CARBON; THE GRAPHITE IN THE NODULAR FORM AND THE EXCESS CARBIDES ARE IN THE FORM OF RANDOMLY DISTRIBUTED, GENERALLY CUBICAL SHAPES, IN A PEARLITIC MATRIX.

. w. H. MOORE ETIAL 3,600,159

Aug. 17, 1971 MODULAR CAST IRON CONTAINING SILICON AND VANADIUM Filed March 11, 1968 FIG. I

INVENTORS FIG United States Patent 015cc 3,600,159 Patented Aug. 17, 1971 US. Cl. 75--123 2 Claims ABSTRACT OF THE DISCLOSURE An improved cast iron particularly adapted for use in articles occupying a high temperature environment and comprising about 3.50% to 5.00% silicon; about 0.70% to 2.00% vanadium; and about 2.80% to 3.80% carbon; the graphite is in the nodular form and the excess carbides are in the form of randomly distributed, generally cubical shapes, in a pearlitic matrix.

Our invention relates to a heat resistant cast iron and, more particularly, to a cast iron containing nodular graphite and silicon and vanadium, in combination, to provide new and useful properties at elevated temperatures.

Industrial engineering machinery is being increasingly subject to more exacting service conditions. In metal forming of such exotic metals as titanium, for example, it is necessary to provide dies which can be used at temperatures ranging from 1200 to 2000 F., as titanium can only be worked effectively in this temperature range. In parts such as supercharges for combustion engines, turboengines, and the like, parts have to withstand heavy mechanical loads at elevated temperatures and must be resistant to thermal creep. This need has led to the development of many heavily alloyed cast irons and steels, which are extremely difficult to cast and to machine and which are very expensive to use.

Our invention has as an object, the production of a relatively inexpensive cast iron alloy having good high temperature characteristics.

A further object is to provide a metal having a high mechanical strength at elevated temperatures in excess of 1200 F.

A further object is to provide a metal which is easily machinable at room temperatures and easily castable in conventional foundry molds.

A further object is to provide a metal which has superior resistance to oxidation, scaling and growth at elevated temperatures.

Other objects of this invention will be apparent on reading the specification and drawings, in which:

FIG. 1 is a photomicrograph at 100 diameters, of the alloy of this invention, showing nodular graphite and dispersed carbides in a pearlitic matrix; and

FIG. 2 is a photomicrograph at 500 diameters, showing the cubic, randomly distributed carbides peculiar to the alloy of this invention.

It is a well known fact that high silicon cast irons are useful for service at elevated temperatures, because of their ability to resist oxidation and scaling and because they exhibit very little dimensional change or growth, when subjected to cyclic heating conditions at elevated temperatures in hostile atmospheric conditions. It is also recognized that these high silicon cast irons containing from 3.50% to 6.50% silicon or more, are extremely brittle at room temperature and, therefore, difficult to cast and fabricate into useful engineering shapes. It has been found that rendering graphite in these cast irons in the nodular form by suitable additions of nodularizing agents such as magnesium, imparts a considerably high degree of mechanical strength, such as, toughness and tensile strength, with additional minor benefits in the ability of the cast iron to perform at elevated temperatures.

We have discovered that the addition of vanadium to high silicon cast iron containing nodular graphite, produces a new and unexpected metallurgical structure and new and unexpected improvement in the ability of this iron to perform satisfactorily at temperatures in the range of 1200 F. to 2000 F. We have found that vanadium renders this cast iron particularly stable, confers a higher tensile strength and hardness at these elevated temperatures, and in no way affects the castability and machinability of parts cast from this alloy.

Vanadium in cast iron has been recognized by those skilled in the art as a carbide former and hardener. In this direction, it is approximately two and one-half times more potent than chromium (another well known hardener used particularly where wear resistance is a factor). In lower silicon irons it is very difficult to prevent the formation of massive brittle carbides in cast iron, even where only relatively small quantities of vanadium, such as 25% or .50% are used. Vanadium additions, therefore, have been found to be limited in usefulness, because they can very easily render the cast iron non-machinable. As most industrial engineering components have to be machined as, for example, in the production of metalforming dies and supercharger parts, it has not previously been considered possible to utilize vanadium to any great extent in such engineering cast irons.

We have found that vanadium may be added to high silicon irons, particularly nodular irons, and that under the conditions set forth herein, excess vanadium is present as randomly distributed carbide particles, which do not affect machinability and which greatly enhance the mechanical properties at elevated temperatures. We

prefer to use a nodular graphite cast iron of the following composition:

Percent Total carbon 2.803.80 Silicon 3.505.00 Manganese 0.20-1.50 Sulphur .025 Phosphorus 10 Vanadium 0.702.00

We do not wish to be limited to any particular composition, with respect to elements such as manganese, sulphur, and phosphorus, which are in the limits normal for cast iron; however, it is natural that, in the case of a nodular cast iron, sulphur content will be low and, in the case of an iron to be used for heat resistance, the phosphorus content will be kept similarly low, to avoid melting out of low melting point phosphides under service conditions. Also, elements such as nickel, copper, tungsten, molybdenum, etc., may be used where certain special effects are desired.

The important feature of our alloy is that the carbon content must not be above 3.80%, because, under these conditions there is a definite tendency to form massive vanadium carbides rather than the dispersed cubic carbides, which are peculiar to the alloy of our invention. Similarly, we do not desire to have the carbon content lower than 2.80%, because of the adverse effect on castability of the alloy. Silicon content, for the purposes of good heat resistance, must be above 3.50% and to prevent excessive brittleness in the final product, we prefer to keep to a maximum silicon content of 5.00%.

We have found that vanadium contents of less than .70%, while useful to heat resistance, do not produce the preferred metallurgical structure containing dispersed cubic carbides, which we feel go hand-in-hand with the improved behavior at high temperatures of the alloy of our invention. We do not know the reason why these carbides are beneficial, except perhaps that, like any carbides they do increase the overall hardness of the metal at high temperatures and their peculiar shape has no effect on either machinability or impact strength, which are both important considerations. It is also very likely that the presence of these carbides indicates a matrix fully saturated with vanadium and having a much higher yield, tensile strength and hardnessparticularly at elevated temperatures. We have found that vanadium contents in excess of 2% tend to produce more massive and uncontrolled carbides and serve no useful purpose in further improvement of the heat resistance for engineering cast irons. The effect of a higher vanadium content on reducing machinability and providing a tendency for brittleness, and the additional high cost of such an alloy, make it impractical to use in amounts above 2%.

As an example of the product of our invention, we prepared a heat having the following analysis:

Percent Total carbon 3.00 Silicon 4.06 Manganese .47 Phosphorus .03 Sulphur .015 Magnesium .035 Vanadium 0.75

The presence of magnesium was necessary for nodularization and the melt was cast into test bars, which were subsequently machined and tested for tensile strength at elevated temperatures.

At the same time we cast a similar alloy having the same bath composition, but without any vanadium being present. The test bars without vanadium were tested for tensile strength on a short-time basis at 1200 F., 1400 F. and 1600 F. These strengths were 9,600 p.s.i., 6,000 p.s.i. and 4,500 p.s.i.

The test bars containing .75% vanadium at the same temperatures; namely, 1200 F., 1400 F. and 1600 F., showed tensile strengths of 18,500 p.s.i., 12,500 p.s.i., and 6,300 p.s.i., respectively.

This shows the remarkable effect of vanadium in increasing tensile strength at elevated temperatures. The structure of the alloy containing the .75 vanadium comprised a fully pearlitic matrix with nodular graphite and contained small cubic particles of vanadium carbide, as illustrated in FIG. 1 of the specification. These results of our alloy containing .75% vanadium compare very favorably with more expensive heat resistant steels and expensive, highly alloyed cast irons containing nickel contents above 14%. A similar cast was made into a die for hot-forming titanium. This die machined very readily and, when placed into service, gave a improved life over high silicon cast iron not containing vanadium, previously used for the same purpose.

We have also found that the presence of vanadium in amounts sufficient to produce the cubical carbides, give an increase of to 100% in Brinell hardnesses at elevated temperatures and it is probable that the increase in service behavior of dies made from this material, is associated with its improved hot hardness characteristics. Similar tests have been run using vanadium in conjunction with small quantities of molybdenum and tungsten and the same improved results have been found. In the manufacture of the cast iron of our invention, we prefer to add vanadium as a vanadium carbon-bearing ferro alloy, because vanadium has an extremely high melting point and is difiicult to incorporate into the melt, if it does not contain carbon.

We find that no essential difference in behavior of our alloy occurs when the nodularity is produced by different means, such as adding nickel, magnesium; magnesium ferrosilicon; injecting pure magnesium; using cerium magnesium calcium alloy, etc. We feel that the improved properties of our alloy are essentially due to the presence of vanadium in the matrix in combination with a silicon content of above 3.50% and the presence also of excess vanadium in the form of small, cubical, randomly distributed carbides in the matrix.

Although we have described our invention in its preferred form with a ceratin degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of the process, modifications in the steps undertaken, and variations in the materials used, may be resorted to, without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An improved heat resistant cast iron having a composition consisting essentially of 3.80% to 2.80% carbon, 3.50% to 5.00% silicon, 0.70% to 2.00% vanadium, a small but effective amount of a nodularizing agent and the balance iron; excess carbon being in the form of nodular graphite and randomly distributed, cubically shaped carbides, in a pearlitic matrix.

2. An improved heat resistant cast iron having a composition of 3.80% to 2.80% carbon, 3.50% to 5.00% silicon, 0.70% to 2.00% vanadium, a small but effective amount of a nodularizing agent and the balance iron; excess carbon being in the form of nodular graphite and randomly distributed, cubically shaped carbides, in a pearlitic matrix.

References Cited UNITED STATES PATENTS 2,158,105 5/1939 Burgess 123R 2,781,284 2/1957 Borneman 75l23R 3,411,957 11/1968 Kiyoshi Takashi 75-123R HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75123 

